That is a good question and here is the answer.
They had real-time onsite video, secured e-mail and were speaking with personnel at both compounds over at least two lines of radio transmission and cell phones. There were two drones overhead. All of that was live-streaming to the entire DoD-DC network of sit-rooms around the world including the White House.
IOWs they had more intel coming in real-time than most military actions taken in Afghanistan or Iraq had.
Were the drones surveying the city or basically hovering around the compound. I have seen photos of the building and grounds, and it did not look very defensible to me. Try to find some on line and tell me what you think.
Benghazi Drone Operator Asks Sean Hannity: Why Hasnt Congress Contacted Me? (Audio)
Hannity Benghazi Drone Caller (Benhazi Drones Not Allowed to be Armed)